Comment 52 for bug 320057

While not solving the underlying issue, this might be a good place to document the work-around, as mentioned in <http://exain.wordpress.com/2008/07/29/thunderbird-active-directory-ldap-lookups-hang-issues/>:
Instead of the standard port 389, specify port 3268, and the freezing is gone.

Successfully tested with Thunderbird 2.0.0.19 on Gentoo Linux against AD on MS Windows Server 2003.